The present invention relates generally to broadband networks, and more particularly to a system for efficiently minimizing the effects of a time-duplex noise environment causing alternate near end crosstalk and far end crosstalk noise in an ADSL system.
Asymmetric digital subscriber line (ADSL) technology has been introduced into the field of broadband networking, among other reasons, to overcome issues faced by traditional voice band technology. Such issues include, but are not limited to, bandwidth limitations. ADSL technology utilizes the infrastructure already in place in a public switched telephone network (PSTN), including copper loops, constructed of copper wires, between a customer premise and a central office. Advantageously, ADSL technology does not require replacement of network equipment such as routers, switches, firewalls and web servers, which are commonly used in today""s paradigm for broadband access.
Unfortunately, in a telephone network, while electrical energy is transmitted across copper wires, a modulated signal also radiates energy onto adjacent copper wire loops that are located in the same cable bundle. This cross coupling of electromagnetic energy is referred to as crosstalk.
In a typical telephone network, multiple insulated copper wire pairs are bundled together into a cable called a cable binder. Adjacent systems within a cable binder that transmit or receive information in the same range of frequencies can create significant crosstalk interference. The crosstalk interference is attributed to crosstalk-induced signals combining with signals that were originally intended for transmission over the copper wire loop. The result is a slightly different shaped waveform than was originally transmitted, which is representative of data degradation.
Crosstalk can be categorized in one of two forms. Near end crosstalk, commonly referred to as NEXT, is the most significant because a high energy signal from an adjacent system can induce relatively significant crosstalk into the primary signal. In other words, NEXT essentially is a measure of the crosstalk noise that two devices used for communication purposes induce upon each other at the same end of the cable binder.
Another form of crosstalk is far end crosstalk, or FEXT. FEXT is typically measured by applying a test signal to a wire pair at one end of a channel and measuring the disturbance on other wire pairs at the far end. Therefore, FEXT essentially measures the crosstalk noise that communications equipment creates for devices it is communicating with at the opposite end of the wire pair used for data transmission. Typically, FEXT is less of an issue than NEXT since the far end interfering signal is attenuated as it traverses the copper wire loop.
In an environment, where Time Compressed Multiplex Integrated Service Digital Network (or TCM-ISDN) is deployed, the source of NEXT and FEXT noises are commonly referred to as TCM-ISDN interferers. The TCM-ISDN system performs alternatively upstream and downstream transmission of data during a period referred to as the TCM timing reference (TTR). In the first half of the TTR, an ISDN central office (ISDN-CO) transmits data to an ISDN remote terminal (ISDN-RT), while during the second half of the TTR period the ISDN-RT transmits data to the ISDN-CO. Consequently, an ADSL transceiver, connected to the wire loop via the CO end, receives NEXT noise from the ISDN-CO during the first half of the TTR, and FEXT noise from the ISDN-RT during the second half of the TTR. Conversely, an ADSL transceiver unit, connected to the wire loop via the ISDN-RT end, receives FEXT noise from the ISDN-CO during the first half of the TTR period, and NEXT noise from the ISDN-RT during the second half of the TTR period.
The effect of crosstalk is more dominant on the ISDN-CO""s side of copper wire loops than on the ISDN-RT side. This phenomena is due to the fact that more copper wires, each of which introduces a crosstalk component, are combined in large bundles as they get closer to entering the ISDN-CO. Conversely, as the loop from the ISDN-CO to the ISDN-RT user is traversed, the loop tends to branch off for connection to numerous ISDN-RTs, resulting in fewer copper wire loops located in a bundle. Therefore, less aggregated NEXT crosstalk is introduced by the ISDN-RT transmitters at the far end wire bundles.
The presence of ISDN-NEXT and FEXT noises affects the performance of an ADSL system differently. During the initialization of a typical ADSL transmission link, an ADSL system estimates the noise level that affects the transmission link. Since NEXT noise is generally very powerful, its presence during bit-loading computation of the ADSL transmission link causes a very poor data rate throughput, which is particularly noticeable on long loop lengths. Further, the noise level measured for bit-loading computation is required to be constant such that a bit loading profile, which ensures a certain bit error rate (BER) with a given noise margin, can be derived and applied for accurate transmission of data.
Unfortunately, as previously demonstrated, in the TCM-ISDN case, noise level is time varying across the TTR period. If noise estimation is implemented with the assumption of a constant stationary noise, the estimation procedure only provides a single average noise level for both FEXT and NEXT noises. Eventually, the noise level computed in this manner is over-estimated when the channel is affected by FEXT noise and underestimated when the channel is affected by NEXT noise. As a result, the transmission of data with a bit loading profile derived from the average noise level, more than meets the requirements during the FEXT period, while it is not possible to ensure transmission of data at the required BER with the desired noise margin during the NEXT period.
In light of the foregoing, the invention generally relates to an ADSL system for operating in a time duplex system that provides alternative configurations for limiting crosstalk in a broadband network by maximizing the bit rate at which information is transmitted, regardless of network topology. The preferred embodiment derives a composite signal to noise ratio from a minimum far end crosstalk signal to noise ratio and a minimum near end crosstalk signal to noise ratio. A maximum bit rate for the transfer of information, which is directly related to the derived composite SNR, is then determined. The information is then transmitted simultaneously between an ADSL-CO and an ADSL-CP at the determined maximum bit rate.
In accordance with an alternative embodiment of the invention, a selection is made between a single, dual, or trial bitmap profile configuration as a function of noise environment and channel loop length, thereby ensuring maximum bit rate throughput of an ADSL system operating in the presence of TCM-ISDN interference.
The invention has numerous advantages, a few of which are delineated hereafter as examples. Note that the embodiments of the invention, which are described herein, possess one or more, but not necessarily all, of the advantages set out hereafter.
One advantage of the invention is that it allows the maximization of the bit rate throughput of an ADSL system operating in the presence of TCM-ISDN interference.
Another advantage is that it allows continuous transmission of data with an ensured transmission performance in any type of network topology, whatever the level of FEXT and NEXT noises can be relative to each other.
Other features and advantages of the present invention will become apparent to one of reasonable skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.